Newsgroups: comp.robotics
Path: brunix!news.Brown.EDU!noc.near.net!MathWorks.Com!europa.eng.gtefsd.com!howland.reston.ans.net!pipex!sunic!ericom!cnn.exu.ericsson.se!s08a04!exujet
From: exujet@exu.ericsson.se (Jerry Ethridge)
Subject: Re: Looking for cheap, slow, low-res solid-
Message-ID: <1994Jan14.205547.5618@exu.ericsson.se>
Sender: news@exu.ericsson.se
Nntp-Posting-Host: s08a04.exu.ericsson.se
Reply-To: exujet@exu.ericsson.se
Organization: Ericsson Network Systems, Inc.
References: <jimn8CJL0G0.M6y@netcom.com>
Date: Fri, 14 Jan 1994 20:55:47 GMT
X-Disclaimer: This article was posted by a user at Ericsson.
              Any opinions expressed are strictly those of the
              user and not necessarily those of Ericsson.
Lines: 51

In article M6y@netcom.com, jimn8@netcom.com (Jim Nitchals) writes:
>csulo@csv.warwick.ac.uk (Mr M J Brown) writes:
>
>>Any other refs on articles for doing this trick? I've tried it and I couldnt
>>read diddly from the chip. It was a working chip tho, you could put it back
>>in the Apple and it worked ... as long as the lights were off.
>
>I believe the tricks were: don't let the chip be auto-refreshed by
>the system; and write a "1" (or a "0"?) then wait a little while, then
>read the result.  If it was being hit by light, the value would change
>to a "0" (or was it..?).  The longer your delay before reading, the
>lower the light level needed to change the bit value.
>
>So:  [fill RAM chip with constant pattern]
>     [wait a short while]
>     [read RAM chip]
>You now know which pixels are brightly illuminated.
>     [fill again]
>     [wait a *longer* period of time]
>     [read again]
>You now know which pixels are less-well lit.
>
>Repeat the process until you've run out of time, or you're satisfied
>that the remaining pixels are too dimly lit to change value.
>
>I don't know where to find cool RAM chips with lids anymore.  You'll
>probably have to scavenge older PC/XT clones to find good ones (as
>you noted, you'll want at least a 4164 for "high res".)
>

I've talked to a professor who has done this and he said that you
write "1" to all locations, then read the results at uniform time periods.
The number of times that a 1 was read before it changes to 0 is an indication
of how much light is falling on that memory bit. If alot of light is falling
on that bit, then you may only read a one in that location 5 times before it
changes to a 0, however on another dimmly lit memory bit, you may read a 1 
for 200 reads before it changes to a 0. The times between reads is a function
of the chip, lens, light level, etc. It is usually found with experimentation.

Hope that helps,
Jerry

===============================================================================
Jerry Ethridge		     Richardson, Tx.                 |
ERICSSON NETWORK SYSTEMS                                    /O\
exujet@exu.ericsson.se                            \_______[|(.)|]_______/
                                                    o   ++   O   ++   o
===============================================================================



